Recently, modified silicon nitride ceramics have been found useful as cutting tools. For such use, the ceramic can be prepared by hot pressing to full density either (a) a loose powder mixture of silicon nitride and powder additives (see U.S. Pat. Nos. 4,264,548; 4,264,550; and 4,323,325), or (b) a semidense billet of a previously heat treated powder preform having crystallized secondary phases (see copending U.S. application Ser. No. 444,251 now abandoned). Because hot pressing takes place under an unusually reactive environment, along with high pressure (2000-6500 psi) and temperature (1500.degree.-1800.degree. C.), it has been necessary to arrange the pressing die to press only a unitary body of the ceramic in a simple geometrical form such as a cylinder or disc. The resulting disc or cylinder must then be cut or sawed with expensive diamond materials to produce a production type cutting tool of appropriate geometry capable of being used in milling and other industrial machine cutting operations.
Producing accurate tool surfaces in their substantially usable condition, as a direct result of hot pressing, has long been a goal of the prior art. However, due to volume changes, and chemical reactions during hot pressing, the goal has not been attained. Considerable removal of material is shaved or ground off the hot pressed product to define the cutting tool insert accurately.
Thus individualized pressing of one body has been the normal commercial mode of producing silicon nitride ceramics which can be formed into a cutting tool. The prior art has considered the goal of making a plurality of cutting tool bodies, each having a desired rough final configuration as a result of hot pressing, within a single hot pressing sequence. This would be most desirable because the unit cost of each tool would be substantially reduced and much of the diamond cutting or shaping would be eliminated. One attempt by the prior art to achieve such simultaneous pressing of multiple independent bodies is represented in British Pat. No. 1,405,171, wherein a plurality of independent triangular modules of silicon nitride material, along with triangular shaped packing bodies deployed along the periphery of the pressing chamber, were nested within a hot pressing cavity and hot pressed under a single stroke or sequence. The number of bodies in the pressing sequence were laid in a common layer limited to two bodies in such layer. In an alternative suggestion, two layers were employed with two bodies in each layer. Since each body was free to move relative to the other when placed in the hot pressing cavity, and since there was no isostatic pressure transfer medium between the bodies, it was difficult to provide for uniform transfer pressure between the bodies while under the pressing sequence. Thus nonuniform densities resulted within the ultimate pressed bodies.